The observed baryon asymmetry of the Universe (BAU), typically quantified in terms of the baryon-to-photon ratio \(\eta _B^{\rm obs} = n_{\rm b}/n_\gamma = \left(6.12 \pm 0.04\right) \times 10^{-10}\)  [1]}, [2]}, cannot be created within the Standard Model (SM) and hence provides clear evidence for new physics.
One of the most attractive possibilities to explain the origin of the BAU consists in baryogenesis via leptogenesis [3]}, which naturally occurs in the type-I seesaw extension of the Standard Model [4]}, [5]}, [6]}, [7]}, [8]} and thus establishes a close connection between early-Universe cosmology and neutrino physics [9]}, [10]}, [11]}.
The main idea behind leptogenesis is to employ the \(CP\) -violating couplings of right-hand neutrinos (RHNs) in order to generate a primordial lepton asymmetry — either via RHN decays [3]} or oscillations [13]} — which is then partly reprocessed into a baryon asymmetry in consequence of the chemical transport in the SM plasma.
Here, a key role is played by the weak sphaleron processes [14]}, which violate baryon-plus-lepton number \(B\!+\!L\)  and hence allow for the generation of nonzero baryon number \(B\) .
Meanwhile, the generation of a primordial lepton asymmetry during leptogenesis can also be regarded as a violation of baryon-minus-lepton number \(B\!-\!L\) , i.e., the linear combination of global charges that is orthogonal to \(B\!+\!L\) .
Unlike other baryogenesis scenarios, such as, e.g., GUT baryogenesis in the standard \(SU(5)\)  grand unified theory (GUT) [15]}, [16]}, [17]}, [18]}, [19]}, leptogenesis therefore does not suffer from disastrous sphaleron wash-out.
Instead, the initial \(B\!-\!L\)  asymmetry remains conserved throughout and sets the scale for the final \(B\)  and \(L\)  asymmetries at the time of sphaleron freeze-out during the electroweak phase transition (EWPT).
